This invention relates to forging and more specifically to methods for making a component part of two dissimilar non-weldable materials. In particular, the invention relates to a forging process for producing a bi-metal mechanical joint between a forged titanium member and a member made of a dissimilar metal.
In aircraft and aerospace industries composite parts made from dissimilar metals are often used. A typical example is a titanium turbine wheel disc mounted on a hardened steel shaft. Currently the titanium disc is bolted to the steel shaft. The hole in the center of the titanium disc reduces its structual integrity and therefore, the thickness of the disc has to be increased to maintain the operating stresses at an acceptable level. The current state of the art for welding dissimilar metals, such as titanium and steel, results in a brittle joint which is seldom sructurally useful and is incapable of carrying a reasonable load.
The known prior art teaches either using a relatively soft cold workable material and a relatively hard material for making mechanical joints between two dissimilar materials, or when both parts to be joined are of a hard material, heating the part to be deformed. In the latter case, the mating portions of the two parts to be joined need to be machined to close tolerances, so that a minimum of deformation of the heated part is required.
It is, therefore, an object of the present invention to provide a joint between two dissimilar metal parts in which one of the parts is forged during the formation of the joint. The deformed part must remain mechanically secure within the non-deformed part in such a way as to avoid looseness or fretting between the joined parts. Since the non-deformed part remains with the formed part when the joint is made, it is important that the interface of the two parts include materials which retard or prevent dissimilar metals corrosion and do not otherwise create problems during the lifetime of the part. On the other hand, it is important that steps be taken to avoid oxidation, which would occur during the forging operation with the titanium and with any other active metals forming the joint. It is also desired to provide a joint between titanium and dissimilar metals in which the size of the joint is reduced over that of the prior art and requirements for further fastening techniques in the joint are reduced.